Two-thirds of people recently infected with the Omicron variant say they had already had Covid previously, the BBC reports.
The findings come from a large, continuing study, React, swab-testing thousands of volunteers in England.
More work is needed to know how many are true reinfections – but the results reveal the groups that appear to be more likely to catch Covid again.
They include healthcare workers and households with children or lots of members under one roof.
More than two million people have been tested in the study.
The latest findings, for the first two weeks of 2022 – round 17 – are based on about 100,000 polymerase chain reaction (PCR) tests posted to volunteers and then returned.
About 4,000 were positive, by far the highest rate seen since the pandemic began.
And when a selection of them were sequenced to check what type of Covid was to blame, virtually all were Omicron – the highly infectious variant, first identified in South Africa, causing a big winter wave of infections in the UK.